This invention relates to the limitation of excessive armature current in a wound armature electric motor and particularly to such limitation by polymer PTC elements placed in series with the motor armature to carry the armature current. Such polymer PTC material has a resistance below a trip temperature sufficiently low as to not significantly affect the armature current, the resistance rising sharply and greatly above the trip temperature.
Polymer PTC materials, which may have a cold resistance on the order of 0.5 ohm or less and a very sharp resistance change at the trip temperature, are more suitable than other PTC materials such as barium titanate for use directly in series with a motor armature, since they have a comparatively low voltage drop and power dissipation. The material is heated directly by the armature current therethrough and trips when the heat generated by the armature current causes the temperature thereof to increase to the trip temperature. However, since the trip temperature is essentially constant, the trip time for an excessive armature current depends on the starting temperature of the PTC material. Such starting temperature is generally the ambient temperature, which may vary greatly in some environments such as motor vehicles.
The trip time following the onset of an excessive armature current must be short enough at low ambient temperatures to allow an increase in resistance in time to protect the motor. However, the trip time at high ambient temperatures must be sufficiently long to prevent nuisance tripping on short transient overcurrents that are not damaging to the motor. The problem is illustrated in FIG. 3, which shows the variation of resistance over time for a single typical prior art polymer PTC element carrying an excessive motor armature current. It can be seen that the trip time is highly variable inversely with ambient temperature. It is desirable to reduce this variability.